Golf club head manufacturers and designers are constantly looking for ways to improve golf club head performance, which includes the forgiveness of the golf club head, while having an aesthetic appearance. Generally, “forgiveness” can be defined as the ability of a golf club head to reduce the effects of mishits, i.e., hits resulting from striking the golf ball at a less than an ideal impact location on the golf club head, on the shot shape and distance of a golf ball struck the by club.
Golf club head performance can be directly affected by the moments of inertia of the club head. A moment of inertia is the measure of a club head's resistance to twisting about the golf club head's center of gravity upon impact with a golf ball. Generally, the higher the moments of inertia of a golf club head, the less the golf club head twists at impact with a golf ball, particularly during “off-center” impacts with a golf ball, the greater the forgiveness of the golf club head and probability of hitting a straight golf shot. Further, higher moments of inertia typically result in greater ball speed upon impact with the golf club head, which can translate into increased golf shot distance.
In general, the moment of inertia of a mass about a given axis is proportional to the square of the distance of the mass away from the axis. In other words, the greater the distance of a mass away from a given axis, the greater the moment of inertia of the mass about the given axis. Accordingly, golf club head designers and manufacturers have sought to increase the moment of inertia about one or more golf club head axes, which are typically axes extending through the golf club head center of gravity, by increasing the distance of the head mass away from the axes of interest.
In an effort to increase the forgiveness of a golf club head, some golf club head manufacturers have focused on the size of the golf club head striking surface. Generally, the larger the striking surface, the greater the forgiveness of the golf club head. However, to maintain the durability of the striking surface, increasing the size of the striking surface typically requires increasing the thickness of the face, e.g., face plate, defining the striking surface, which has a direct effect on the Coefficient of Restitution (COR) of the striking surface, or the measurement of the ability of the striking surface to rebound the ball, e.g., the spring-like effect of the surface. In a simplified form, the COR may be expressed as a percentage of the speed of a golf ball immediately after being struck by the club head divided by the speed of the club head upon impact with the golf ball, with the measurement of the golf ball speed and club head speed governed by United States Golf Association guidelines
United States Golf Association (USGA) regulations and constraints on golf club head shapes, sizes and other characteristics tend to limit the moments of inertia and COR achievable by a golf club head. According to the most recent version of the USGA regulations, golf club heads must, inter alia, be generally plain in shape, have envelope dimensions at or below maximum envelope dimensions (maximum height of 2.8 inches, maximum width of 5.0 inches and a maximum depth of 5.0 inches), and have a volume at or below a maximum head volume of 470 cm3. It should be noted that this maximum volume constraint of 470 cm3 is well below the volume of the maximum envelope dimensions. Note that the 470 cm3 USGA limit includes a 10 cm3 tolerance (i.e., 460 cm3+10 cm3). Further, the USGA regulations require the COR value to be less than 0.830, or have a Pendulum Characteristic Time (PCT) of less than 257 microseconds. The COR and PCT limits just identified each include a tolerance.
Often, golf club manufacturers are faced with the choice of increasing one performance characteristic at the expense of another. For example, to promote forgiveness, some conventional golf club heads focus on increasing the moments of inertia at the expense of increased striking surface size. In these golf club heads, as much of the golf club head mass as possible is moved away from the center of gravity. However, due to mass constraints resulting from attempting to achieve the desired swing weight (e.g., driver club head mass typically ranges from about 185 g to about 215 g), the more mass that is distributed away from the center of gravity, the less mass available for the face. With less mass available for the face, to remain within the USGA constraints governing COR and PCT, the golf club head face thickness, and thus the club head striking surface size, is limited. Accordingly, with these conventional golf club heads, the forgiveness of the heads can be increased by the increased moments of inertia, but limited by the resulting constraints on the size of the golf club head striking surface.
Conversely, to promote forgiveness, some conventional golf club heads focus on increasing the size of the golf club head striking surface at the expense of increased moments of inertia, potentially also sacrificing desired center-of-gravity (“CG”) properties. As described above, with conventional face designs, the larger the size of the striking surface, the thicker and more massive the face must be to comply with USGA constraints. With more mass dedicated to the face, there is typically more mass closer to the center of gravity, and less mass, e.g., discretionary mass, available for moving away from the center of gravity. Accordingly, with these conventional golf club heads, the forgiveness of the heads can be increased by the increased striking surface sizes, but limited by the resulting constraints on the achievable moments of inertia.
As described above, golf club designers and manufacturers have struggled to design USGA-conforming golf club heads that have both high moments of inertia and large striking surface sizes for improved forgiveness.